It is known that in many industrial processes it is necessary to control the specific amounts of gas and gas mixtures delivered to point-of-use locations with a high degree of accuracy. Particularly, in semiconductor processing it has become increasingly important to control the specific mass of gases delivered during the fabrication of semiconductor devices. As the speed of next generation semiconductor devices increases, and the size/dimension of next generation semiconductor devices decreases, the degree of accuracy and control over the fabrication of next generation semiconductor devices must increase. As the architecture of semiconductor devices falls below the three submicron scale the semiconductor industry must find more accurate methods for delivering specific amounts of gas to a process chamber. The use and benefits of the present invention are described herein in relation to semiconductor processing, and more specifically, to the delivery of a gas to a process chamber. It is appreciated, however, that such a description is merely illustrative and that the present invention is applicable in other fields where it is desired to precisely control the amount of gas being delivered to a point-of-use location.
One prior art method for delivering a gas to a semiconductor process chamber includes the use of a Mass Flow Controller (MFC). FIG. 1 illustrates a prior art Mass Flow Controller (MFC) that is used to control gas flow. The MFC is calibrated to deliver a specific mass of gas to a process chamber within a specified amount of time. For example, an MFC may be calibrated to deliver 100 standard cubic centimeters per minute (sccm) of nitrogen gas (N.sub.2) to a process chamber.
In order to control flow of gas the MFC divides the flow of gas between heated sensing tube (sensor) 110 and flow restriction bypass (bypass) 120. The MFC divides the flow of gas such that a majority of the gas flows through bypass 120 and only a small portion of gas flows through sensor 110.
Mass flow is measured in sensor 110. As the gas flow passes through heater coil (coil) 111 the gas picks up and carries heat toward heater coil (coil) 112. The movement of heat by the gas develops a temperature difference between the two coils. Coils 111 and 112 are both heaters but also act as resistance temperature detectors (RTDs) that measure the temperature of the gas. Thus, as the gas flows between coil 111 and coil 112, the change in temperature between coil 111 and coil 112 is measured and can be correlated to the mass flow rate of the gas by the MFC control system 130.
Once the temperature difference is measured and the correlating mass flow rate of the particular gas is determined, control system 130 adjusts the position of control valve 140. The position of control valve 140 is set in order to obtain the desired (or calibrated) flow rate for the particular gas being used.
One problem with the method associated with the MFC for delivering a specific mass of gas to a process chamber is the degree to which the MFC method is accurate. MFC's are currently designed to run at 40 to 80% of their actual calibrated flow rate with an accuracy level of approximately 5%. For example, with respect to the MFC for N.sub.2 calibrated with a flow rate of 100 sccm, described above, that particular MFC is designed to deliver N.sub.2 at flow rates in the range of 40-80 sccm. Outside the 40-80% range the accuracy level of the MFC falls off. Next generation semiconductor devices require fabrication processes with greater accuracy than the prior art MFCs.
Another problem associated with MFCs is that they offer a limited dynamic range. The dynamic range is the ratio of the maximum and minimum controlled flow rates. As mentioned above, most MFCs are designed to run at 40 to 80% of their actual calibrated flow rate to achieve an accuracy of approximately 5%. As such, the dynamic range of such devices is limited to a ratio of approximately 2 to 1.
Another problem with the MFC is that most processes require that the flow of gas to the process chamber has the ability to be controlled. In many applications, it is not desirable to deliver all of the gas for a particular recipe to the process chamber all at once. Likewise it may not be desirable to place a small portion of the gas into the process chamber at the beginning of the process and a larger portion of the gas into the process chamber at the end of the process (or vice versa). Instead it is desirable to deliver the gas to the process chamber at a controlled rate in a manner that optimizes the productivity of that process. Because the accuracy of the MFC decreases outside the 40-80% range of the particular MFC's calibrated flow rate, the degree of control over the delivery of the gas also decreases.
Other methods and apparatus are used to deliver gases to point-of-use locations. For example, the use of a variable flow valve under the control of feed-back control loop is a common method for controlling the delivery of a gas to a point-of-use location. FIG. 2 illustrates a typical prior art gas delivery system employing a variable flow valve 210 under the control of a feed-back controller 230. Gas is delivered from a gas source to a point-of-use location 250 by adjusting the throat area of valve 210 in response to a control signal 234. Control signal 234 is generated in response to a comparison between a desired flow input signal 236 and a measured flow signal 220. The desired flow input signal is generally provided through a user interface or from a preprogrammed process recipe. Measured flow signal 220 is produced by a flow meter or other flow measuring device 220 located at a point downstream of variable flow valve 210.
Over time the flow constant (C.sub.v) of variable flow valve 210 changes due to wear or deposit build-up. In addition, output signal 236 of flow measuring device 220 changes over time for any given measured flow due to a phenomenon known as "drift." The change in flow constant, C.sub.v, and the occurrence of "drift" both act to reduce the accuracy of the flow control apparatus. As a result, currently available feed-back flow control devices require the frequent implementation of time consuming recalibration procedures to maintain the devices within acceptable accuracy ranges. Such calibration procedures are costly in that they result in process down time and require the use of well-trained technicians to perform the procedure.
Thus, what is needed is a method and apparatus that is capable of delivering a gas flow to a processing apparatus with a high degree of accuracy over a large dynamic range.